Polysuccinimide is a precursor of polyaspartic acid. Both have been formed by a number of methods known in the art. The term polyaspartic acid as used herein refers to the free acid and to the salts of polyaspartic acid. Polyaspartic acid is suitable as an antideposition agent, and as a plant nutrient uptake facilitator. Polysuccinimide itself is useful as a builder for detergent compositions, and also as an antideposition agent.
It is known that relatively high molecular weight polysuccinimide can be produced by first reacting maleic anhydride, water and ammonia and then polymerizing that reaction product with a preformed polysuccinimide at a higher temperature (Koskan et al. U.S. Pat. No. 5,219,952). Thereafter the polysuccinimide can be base hydrolyzed to polyaspartic acid, if desired.
U.S. Pat. No. 5,116,513 to Koskan et al. teaches the thermal polymerization of aspartic acid to polysuccinimide, which is then hydrolyzed to polyaspartic acid. It is also known that polyaspartic acid can be produced by reacting maleic anhydride, water and ammonia at a molar ratio of maleic anhydride to ammonia of 1:1 to 1:1.5 at 120.degree. to 150.degree. C. (Boehmke U.S. Pat. No. 4,839,461).
Such prior art processes suffer from various disadvantages, however. For one thing, they are capital intensive, energy intensive, and time consuming because the maleic anhydride is mixed with water at elevated temperature, the ammonia is added in a manner minimizing ammonia loss, and the water of solution is evaporated with vacuum or heating. Also, expensive ammonia handling equipment is needed at all stages of the reaction.
The art needs a new and improved method for producing polysuccinimide from maleic anhydride and ammonia which overcomes these disadvantages.